The southern platyfish, common platy, moonfish or mickey mouse platy (Xiphophorus maculatus) is a species of freshwater fish in family Poecilidae of order Cyprinodontiformes. A live-bearer, it is closely related to the green swordtail (X. helleri) and can interbreed with it. It is native to an area of North and Central America stretching from Veracruz, Mexico, to northern Belize.
The southern platyfish grows to a maximum overall length of 6.0 cm (2.4 in). Sexual dimorphism is slight, the male's caudal fin being more pointed. The anal fin of the male fish has evolved into a gonopodium, a stick-shaped organ used for reproduction. The female southern platyfish's anal fin is fan shaped. Wild varieties are drab in coloration, lacking the distinctive dark lateral line common to many Xiphophorus species.
X. maculatus prefers slow-moving waters of canals, ditches, and warm springs. Omnivorous, its diet includes both plants and small crustaceans, insects, and annelid worms.
Breeders have developed a multitude of color varieties (e.g. orange, red, yellow, red/black, and black/white) which are common aquarium fish for hobbyists.
The southern platyfish is commonly known simply as the platy (pl. platys or platies), from the fish's original generic name, Platypoecilus.